A means system is known for reinforcing the loading capacity of a tractor vehicle, making it possible for it to be hitched under a semi-trailer for transporting heavy loads, the system consisting in the addition of a dolly which is itself similar to a very short semi-trailer hitched to the pulling vehicle. The dolly in turn receives in its center, by means of a second hitching system, the front part of the semi-trailer for transporting heavy loads.
Thus, an assembly of three successive elements is obtained articulated two by two about a vertical shaft, comprising two articulation points generally formed by means of automatic couplings.
In a known embodiment, the dolly wheels are not steerable. The dolly travels in the tracks of the tractor when the assembly is travelling in a straight line and remains close to the inside of the turn when the assembly negotiates a curve. Rearward travel is possible, but it relies on the skill of the driver.
In a second known improved embodiment, the wheels of the dolly are steerable from a so-called "independent" control system which is generally only used for particular maneuvers and particularly for rearward travel because it requires the presence of a special driver, in charge of this control.
In other known embodiments, the wheels of the dolly are steerable from a so-called "automatic" control system, therefore requiring no driver. These systems are all based on the detection of the relative angle about the vertical axis of the articulation between the dolly and the tractor which is in the form of a curve and which increases inversely to the radius of the curve. They are generally designed to cause the wheels of the dolly to follow the track of the non steerable wheel trains of the tractor.
These known systems do not operate in rearward travel, because the more the curve of evolution decreases, the more the angle increases which causes an increase of the angle of the steerable wheels of the dolly, resulting in rearward travel, in further reducing the curve of evolution of the dolly which therefore rapidly skews with respect to the tractor. In order to provide rearward travel it is necessary to provide, in addition, a second control for steering the dolly, called independent, and to go over from automatic steering to independent steering. It should be noted that the rearward travel maneuvers are important in articulated trucks for transporting heavy loads, for very often the semi-trailer itself has its front part in the form of a swan-neck, which can be uncoupled from its platform to allow easy loading of the load and the front part must then move back before recoupling.
It is further to be noted that for certain maneuvers, the driver of the tractor desires to change direction on the spot, which he can do with the steering wheels of his tractor but then the steering wheels of the dolly do not move, and it is only when the tractor advances that the procedure: formation of the curve of evolution of the tractor on the ground, formation of the relative angle between the dolly and the tractor then changing the steering direction of the wheels of the dolly, will take place. This delay may be prejudicial with respect to the desired maneuver.